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Growth and form of the mound in Gale Crater, Mars: Slope-
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wind enhanced erosion and transport
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Authors: Edwin S. Kite1* and Kevin W. Lewis 2
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Affiliations:
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1
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91125, USA.
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2
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*Correspondence to: [email protected]
Geological & Planetary Science, California Institute of Technology, MC 150-21, Pasadena CA
Department of Geosciences, Princeton University, Guyot Hall, Princeton NJ 08544, USA.
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Abstract: Gale crater, the landing site of the Curiosity Mars rover, hosts a 5 kilometer high
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layered mound of uncertain origin which may represent an important archive of the planet's past
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climate. Although widely considered to be an erosional remnant of a once crater-filling unit, we
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combine structural measurements and a new model of formation to show how this mound may
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have grown in place near the center of the crater under the influence of topographic slope-
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induced winds. This mechanism implicates airfall-dominated deposition with a limited role for
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lacustrine or fluvial activity in the formation of the Gale mound, and is not favorable for the
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preservation of organic carbon. Slope-wind enhanced erosion and transport is widely applicable
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to a range of similar sedimentary mounds found across the Martian surface.
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Main text: In early August 2012 NASA’s Mars Science Laboratory (MSL) rover will land in
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Gale Crater, Mars. MSL will target a 5km-tall layered sedimentary mound within Gale that is
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believed to archive long-term climate change on Mars (1). Most of Mars’ sedimentary rocks are
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in the form of intra-crater or canyon mounded deposits like the Gale mound. For example, in the
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30km-diameter Endeavour Crater recently entered by the Mars Exploration Rover Opportunity
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(2), a thin tongue of sedimentary sulfates is three-quarters encircled by a moat. Intra-crater
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mound deposits are not restricted to low latitudes. Radar sounding of crater-filling ice mounds
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near the north polar ice sheet shows that they grew from a central core, and are not relicts of a
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formerly more extensive ice sheet (3). However, identifying the physical mechanism(s) that
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explain the growth and form of equatorial crater mounds has been challenging, in part because
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these deposits have no clear analog on Earth. The current prevailing view on the formation of
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intra-crater mounds is that sedimentary layers completely filled each crater at least to the summit
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of the present-day mound deposit. Subsequent erosion, decoupled from the deposition of the
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layers, is invoked to explain the present-day topography. If the sedimentary rocks formed as
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subhorizontal layers (4-5), in an evaporitic playa-like setting (6), then >>106 km3 must have been
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removed to produce the modern moats and mounds (7-8). This may cause problems with global
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S mass balance (9). This scenario predicts near-horizontal or slightly radially-inward dipping
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layers controlled by surface or ground water levels. To test this prediction, we made multiple
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measurements of bedding orientation from four separate 1-m scale stereo elevation models from
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the High-resolution Imaging Science Experiment (HiRISE) camera, produced by Caltech and the
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USGS via the method of Ref. 10. To the contrary, our new structural measurements from
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multiple sites around the base of the Gale mound reveal layers with shallow but significant dips
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oriented consistently outwards from the mound center (Figure 1; Table S1), suggesting a revised
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formation mechanism.
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Our goal in this paper is to develop the simplest possible model that can account for the
shape and stratigraphy of Mars' sedimentary rock mounds. We assume layers accumulate by the
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accretion of atmospherically transported sediment, and are destroyed by slope-wind erosion
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(Figure 2). Slope-wind enhanced erosion and transport (SWEET) of indurated or lithified
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sedimentary materials cannot explain layer orientations at Gale unless the moat seen in Figure 1
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was present during mound growth. This implies a coupling between mound primary layer
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orientations, slope winds, and mound relief.
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Mars is a windy place: saltating sand sized particles are in active motion (11), at rates that
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predict aeolian erosion of bedrock at 10-50 µm/yr (12,13). Aeolian erosion of rock has occurred
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within the last roughly 1-10 Ka (14) and is probably ongoing. The inability of General
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Circulation Models (GCMs) to reproduce these observations shows that small-scale winds, not
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the regional-to-global winds resolved by GCMs, are responsible for saltation. Because of Mars’
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thin atmosphere, slope winds dominate the circulation in craters and canyons (15,16).
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Downslope-oriented yardangs, crater statistics, and hematite lags show that the mounds of Valles
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Marineris are being actively eroded by slope winds. Slope-enhanced winds also define both the
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large-scale and small-scale topography of the polar layered deposits (17,18). Aeolian processes
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were also important on early Mars. Most of the observed sedimentary bedforms at the
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Opportunity and Spirit landing sites are sand dune foresets (e.g. 19) and aeolianites likely
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represent a volumetrically significant component of the ancient sedimentary rock record,
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including within the strata of the Gale mound (20). In places like Becquerel Crater and the upper
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portion of the Gale mound, bedding is quasi-periodic and bundled, indicating slow (~30 µm/yr)
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orbitally-paced accumulation (21). These rates are comparable to the modern gross
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atmospherically-transported sediment deposition rate (101-2 µm/yr ; e.g. 22) and to the inferred
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modern aeolian-abrasion rate, suggesting that aeolian processes may be responsible for the
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deposition of the layers. In addition, orbital images of deposits within Valles Marineris show
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strata that are meters-to-decameters thick, laterally continuous, with horizontal-to-draping layer
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orientations, and display very few angular unconformities (e.g., 23). Taken together, slow
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accumulation, rarity of unconformities, and draping layer orientations suggest that sedimentary
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deposits formed by the accretion of atmospherically-transported sediment (ash, dust, impact
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ejecta, ice nuclei, or rapidly-saltating sand) were common on both modern and early Mars (24).
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In one horizontal dimension (x), topographic change dz/dt is the balance of a net
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atmospheric source term D and erosion E:
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dz/dt = D(x,t) – E (1)
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Initial model topography in Figure 2 is a basalt (nonerodible) crater/canyon with a flat
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floor of half-width R and long, 20° slopes. To highlight the role of slope winds, we initially
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assume D is constant and uniform (for example, uniform dust concentration multiplied by a
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uniform settling velocity, with no remobilization). Sediment induration is probably required to
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prevent the atmospheric source term from averaging to zero over long timescales. The necessary
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degree of induration is not large: for example, 6-10 mg/g chloride salt increases the threshold
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wind stress for saltation by a factor of e (25). Shallow diagenesis (26) could be driven by
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surficial snowmelt or rainfall.
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We assume that E depends on the maximum surface shear velocity magnitude, U:
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E = kUα (2)
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where k is an erodibility coefficient and α is an exponent parameterizing the exponent
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relating erosive mass flux to shear velocity. For sand transport, soil erosion, and rock abrasion, α
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~ 3-4 (27-29). We assume eroded material does not pile up in the moat but is instead removed
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from the crater, for example through breakdown to easily-mobilized dust-sized particles (30).
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Shear velocity at x is given by
±!
𝑈(𝑥) = 𝑈𝑜 + max !
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𝜕𝑧′
− 𝑥 − 𝑥 !
exp dx ! 𝜕𝑥′
𝐿
(3)
where Uo is synoptic or background shear velocity, and the integral corresponds to a
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contribution from slope winds. z' is local topography, x and x' are distances from the crater
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center, L is a slope-wind correlation length scale, and the +/- max() operator returns the
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maximum of downslope (nighttime) or upslope (daytime) winds blowing from either the left or
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the right. The slope wind is affected by topography throughout the model domain, but is most
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sensitive to slopes within L of x, consistent with analytic and experimental results (31-32)
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showing that the strongest winds occur on steep slopes. Uo is set to zero in this paper, but large
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Uo does not affect our conclusions. Finally, we define D’ as the deposition rate divided by the
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mean erosion rate on the crater/canyon floor at simulation start.
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Our model is idealized and excludes both processes that require rover measurements
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(such as grain-size controls, and the source of erosive tools), and processes that will require
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future 3D GCM/mesoscale modeling (such as slope wind dependence on paleoatmospheric
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pressure). Additional numerical diffusivity at the 10-3 level is used to stabilize the solution.
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Results for α =3, R/L=2.4, D’ = 0.4 are as follows (Figure 2). Initially (phase I), slope
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winds prevent net accumulation on the crater walls or around the perimeter of the floor. As a
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result, the mound nucleates as a central moat-bounded mesa. Layer orientations in the mesa
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interior are horizontal, steepening near the mesa edge. The mound grows by vertical aggradation
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and can either prograde slightly into the moat or hold position, depending on exact parameter
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values. Aggradation is rapid and aggradation rate does not greatly decrease upsection, consistent
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with observations that do not show a systematic decrease in layer thickness with height (24). The
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dip near the mound flank increases as the mound aggrades vertically. Eventually, slope winds
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down the growing mound are sufficient to prevent net aggradation at the toe, leading to Phase II
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(Figure 2b). During Phase II, mound height continues to increase but mound width decreases due
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to flank erosion. Flank erosion is caused by katabatic winds flowing down the growing mound.
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Erosion propagates updip, exposing beds deposited during Phase I. When erosion reaches the
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mound top, the mound becomes entirely self-destructive (Phase III). Mound height decreases
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because flank retreat leads to greater slopes and, therefore, stronger slope winds. Decreasing the
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mound height reduces the maximum potential downslope wind. However erosion also decreases
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mound width, which helps to maintain steep slopes and correspondingly destructive slope winds.
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Topographic change is much slower during Phase III than during Phases I and II.
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This evolution, which does not require any change in external forcing with time,
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describes a clockwise trajectory on a moat width/mound height diagram (Figure 2b). The
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systematic exposure of layering on the Gale mound’s flanks and upper surface shows that it has
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entered Phase III. Flank erosion during later phases tends to remove any unconformities formed
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near the edge of the mound, while exposing the stratigraphic record of earlier phases for rover
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inspection. Exhumed layers are buried to kilometer depths, but relatively briefly, implying
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limited diagenesis of clays and sulfates (1). During phase I, dz/dt is not much slower than D. If D
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corresponds to vertical dust settling at rates similar to today, then we predict that the lower Gale
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mound accumulated in 107-8 yr, consistent with dates suggesting that the time represented by the
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lower Gale mound is a small fraction of Mars’ history (33).
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Values of L and D’ on Early Mars are not known, but Gale-like shapes and stratigraphy
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arise for a wide range of reasonable parameters (SM Text; Figure S1). Consistent with data
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(Figure S2), moats do not extend to basement for small R/L, and for the largest R/L multiple
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mounds can develop within a single crater.
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D could vary on timescales that are much shorter than the time for growth of the mound,
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for example if Milankovitch cycles set the availability of liquid water for cementation. We set
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D(t) = D(t=0) + D(t=0)cos(nt), where the oscillation frequency n is chosen to be faster than the
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mound growth timescale. Low-angle unconformities can now be preserved, and the likelihood of
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unconformities increases upsection and towards the margin of the moat (Figure 2c). A late-stage
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draping layer of sedimentary rock crosscuts layers within the mound core at a high angle, and is
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itself broken up by further aeolian erosion. Thin mesa units mapped at Gale and more widely on
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Mars have these characteristics (4,20).
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Deposition at a constant long-term-average rate is unrealistic because the rate of
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sedimentary rock formation on Mars is close to zero in the modern epoch, most likely because
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atmospheric loss has restricted surface liquid water availability (34-37). Setting D’ to zero after
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the crater is incompletely filled allows slope winds down the wall to expose layers and form a
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moat, which expands the portion of parameter space that allows moats and mounds to form.
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Because groundwater-limited evaporite deposition would infill moats and produce near-
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horizontal strata, SWEET is incompatible with a global-groundwater water source for early
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diagenesis of sedimentary rocks, and with any deposition mechanism that preferentially fills
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topographic lows (e.g., fluviodeltaic or lacustrine sedimentation). A top-down water source (ice
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weathering, snowmelt, or rainfall) is predicted instead (9,37). Perennial surface liquid water
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would shut down saltation and aeolian erosion. Similar to observations along the Opportunity
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rover traverse (38), we predict long dry windy periods interspersed by brief wet periods at Gale.
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We also predict that the outward-dipping layer orientations in the Gale mound are primary.
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Alternative models for the formation of Gale’s mound involve landslides, preferential dissolution
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of soluble layers near the crater edge, or tectonic tilting, to produce the observed outward dips.
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When MSL lands in Gale Crater, it can immediately begin to collect observations that will test
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our model’s predictions, as well as the alternative hypotheses for the mound’s formation. For
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example, MSL will have the ability to observe evidence for subsurface dissolution within the
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crater if it occurred, while measurements of river-deposit tilts and paleoflow directions (e.g.,
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from clast imbrication) may constrain post-depositional tilting. MSL can also measure present-
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day wind velocity using the Rover Environmental Monitoring System (REMS) instrument, and
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image fossilized aeolian bedforms. Unconformities, if present, should be oriented away from the
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center of the present mound and should be more frequent upsection. Slow, orbitally-paced
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sedimentation and oscillation between reducing and oxidizing conditions would disfavor
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preservation of organic carbon.
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Acknowledgements: We thank Mike Lamb, Claire Newman, Mark Richardson, Bill Dietrich,
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Woody Fischer, and especially Kathryn Stack, for their intellectual contributions.
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Fig. 1. Bedding orientation measurements from four locations around the margin of the Gale
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crater mound. Individual measurements are marked in red, with the average at each site
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indicated by the dip symbol. At each location, beds consistently dip away from the center of the
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mound, consistent with the proposed model. Background elevation data is from the High-
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Resolution Stereo Camera (HRSC) (http://europlanet.dlr.de/node/index.php?id=380), with
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superimposed geologic units from Ref. 39. The MSL landing ellipse is shown in white.
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(A)
343
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(B)
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Fig. 2. Simulated sedimentary mound growth and form. Colored lines in (A) correspond to
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snapshots of the mound surface equally spaced in time (blue being early and red being late). The
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black line corresponds to the initial topography. (B) shows mound geometry, with two colored
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dots for every colored line in (A). I, II, and III correspond to phases in the evolution of the
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mound. (A) and (B) are for steady uniform deposition. Results for time-varying uniform
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deposition appear very similar at this scale.
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(C)
353
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(C) shows stratigraphy resulting from sinusoidally time-varying deposition. For clarity, only a
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small number of oscillations are shown. Color of strata corresponds to deposition rate: blue is
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high D, which might correspond to wet climates (36,37), and red is low D, which might
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correspond to dry climates (36,37). Note low-angle unconformities, and late-stage flanking unit
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intersecting the mound core at a high angle.
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Supplementary materials
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Controls on mound growth and form
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L will vary across Mars because of changing 3D topography (40), and will vary in time because
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of changing atmospheric density. Ref. 41 shows L ~ 20km for Mars slopes with negligible
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geostrophic effects. Simulations of gentle Mars slope winds strongly affected by the Coriolis
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force suggest L ~ 50-100 km (42-43). Entrainment acts as a drag coefficient with value 0.02-0.05
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for Gale-relevant slopes (44-46), suggesting L = 20-50 km for a 1km thick cold boundary layer.
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Therefore L ~ 101-2 km is reasonable, but there is considerable uncertainty, which we explore in
367
the next section using a parameter sweep.
368
To determine the effect of parameter choices on sedimentary rock mound shape, size,
369
stratigraphy and exposure, we carried out a parameter sweep in α, D’, and R/L (Figure S1). Weak
370
slope dependence (α = 0.05) is sufficient to produce strata that dip toward the foot of the
371
crater/canyon slope (like a sombrero hat). Similarly weak negative slope dependence (α = -0.05)
372
is sufficient to produce concave-up fill.
373
At low R/L (i.e., small craters) or at low α, D’ controls overall mound shape and slope
374
winds are unimportant. When D’ is high, layers fill the crater; when D’ is low, layers do not
375
accumulate. When either α or R/L or both are >~ O(1), slope wind enhanced erosion and
376
transport dominates the behavior. Thin layered crater floor deposits form at low D’, and large
377
mounds at high D’. At high R/L and high α (large craters and knickpointlike erosion), multiple
378
mounds form at late time.
379
If L is approximated as being constant across the planet, then R/L is proportional to
380
crater/canyon size. Moats do not extend to basement for small R/L, although there can be a small
381
trench at the break-in slope. For larger R/L, moats form, and for the largest craters/canyons,
382
multiple mounds can develop within a single crater because slope winds break up the deposits.
383
This is consistent with data which suggest a universal length scale for mound size (Figure S2).
384
Small exhumed craters in Meridiani show concentric layering consistent with concave-up dips.
385
Larger craters across Meridiani Planum, together with Gale and Nicholson Craters, the smaller
386
Valles Marineris chasmata, and the north polar ice mounds, show a single mound. The largest
387
canyon system on Mars (Ophir-Candor-Melas) shows multiple mounds per canyon. Steeper wall
388
slopes in the model tend to push mound break-up to larger sizes. Gale-like mounds (with erosion
389
both at the toe and the summit) are most likely for high R/L, high α, and intermediate D’ (high
390
enough for some accumulation, but not so high as to fill the crater) (Figure S1). These sensitivity
391
tests suggest that mounds with erosion both at the toe and the summit are a generic outcome of
392
steady uniform deposition modified by slope-wind enhanced erosion and transport for estimated
393
Early Mars parameter values.
394
395
Sequence stratigraphy on Mars
396
Sequence stratigraphy on Earth divides sedimentary deposits into genetically related packages
397
bounded by hiatuses controlled by sediment supply, subsidence, and changes in base level. On
398
the basis of our model results (Figure 2c), we suggest a Martian equivalent for the deposition of
399
layered deposits in which the primary control is not base-level change, but changes in the ability
400
of the climate system to indurate and cement atmospherically-transported sediment (e.g., 36-37).
401
Rare wet periods provide an opportunity for mound accumulation, and these sequences are
402
bounded by surfaces of nondeposition that represent mound degradation during more common
403
dry periods. These oscillations may be orbitally paced, although the time represented by each
404
sequence is unknown.
405
406
Figure S1. Overall growth and form of sedimentary mounds – results from a model parameter
407
sweep varying R/L and d’, with fixed α = 3. Black square corresponds to the results shown in
408
more detail in Figure 2. Symbols correspond to the overall results:– no net accumulation of
409
sediment anywhere (blue open circles); sediment overtops crater/canyon (red filled circles);
410
mound forms and remains within crater (green symbols). Green filled circles correspond to
411
outcomes where layers are exposed at both the toe and the summit of mound, similar to Gale.
412
Multiple mounds form in some of these cases (see Supplementary Text).
413
414
Figure S2. Width of largest mound does not keep pace with increasing crater/canyon width,
415
suggesting a length threshold beyond which slope winds break up mounds. Blue dots correspond
416
to nonpolar crater data, red squares correspond to canyon data, and green dots correspond to
417
polar ice mound data. Gray vertical lines show range of uncertainty in largest-mound width for
418
Valles Marineris canyons. “G” corresponds to Gale Crater. Craters smaller than 10km were
419
measured using CTX or HiRISE. All other craters, canyons and mounds were measured using
420
the THEMIS global day IR mosaic on a MOLA base. Width is defined as polygon area divided
421
by the longest straight-line length that can be contained within that polygon.
422
Table S1. Layer orientation measurements summarized in Figure 1.
Lon (°)
Lat (°)
Z
Dip (°) Dip Az.
(m)
HiRISE Image ID
(°)
138.3949
-5.0223469
-3263.1
3.53
30.68
PSP_008437_1750
138.39266
-5.0238771
-3201.9
2.52
62.01
PSP_008437_1750
138.38631
-5.0158761
-3216.9
2.10
94.68
PSP_008437_1750
138.38702
-5.0155078
-3201.4
7.31
41.72
PSP_008437_1750
138.39168
-4.9983579
-3554.2
2.06
54.81
PSP_008437_1750
138.3878
-5.0030348
-3429.9
0.43
-21.29
PSP_008437_1750
138.37991
-5.0045169
-3425.8
5.04
89.54
PSP_008437_1750
138.37958
-5.0041789
-3434.5
3.79
70.24
PSP_008437_1750
138.39253
-4.9974918
-3583.8
4.65
51.98
PSP_008437_1750
138.39671
-5.0123743
-3421.5
4.14
47.28
PSP_008437_1750
138.39559
-5.0314236
-3290.1
4.07
40.92
PSP_008437_1750
138.39618
-5.0301064
-3308.2
2.55
76.18
PSP_008437_1750
138.39374
-5.03171
-3260.1
3.24
43.31
PSP_008437_1750
138.39205
-5.0351893
-3176.3
2.07
75.25
PSP_008437_1750
138.3917
-5.0350616
-3173.3
2.21
86.77
PSP_008437_1750
137.49485
-4.6858121
-4098.5
3.34
148.1
ESP_023957_1755
137.49197
-4.6847781
-4103.8
3.98
174.47
ESP_023957_1755
137.48052
-4.6893313
-4101.3
6.06
132.82
ESP_023957_1755
137.53315
-4.6593871
-4120.3
6.48
114.03
ESP_023957_1755
137.5374
-4.6588595
-4108.2
0.50
13.56
ESP_023957_1755
137.53552
-4.6621189
-4073.1
3.90
-149.05
ESP_023957_1755
137.52567
-4.6641024
-4127.1
7.16
130.19
ESP_023957_1755
137.52487
-4.6636562
-4138.3
4.83
152.05
ESP_023957_1755
137.52653
-4.6655284
-4101.1
4.59
-153.08
ESP_023957_1755
137.50615
-4.676802
-4083.2
2.32
83.97
ESP_023957_1755
137.51028
-4.6721369
-4128.0
3.24
91.62
ESP_023957_1755
137.27098
-4.8715007
-3849.6
1.09
10.19
PSP_001488_1750
137.26671
-4.871837
-3857.7
5.19
85.97
PSP_001488_1750
137.27073
-4.8726906
-3833.2
2.61
-51.44
PSP_001488_1750
137.28434
-4.9179523
-3513.9
6.78
136.55
PSP_001488_1750
137.33063
-4.8310273
-3768.9
2.16
140.09
PSP_001488_1750
137.33036
-4.8285655
-3792.5
2.65
143.84
PSP_001488_1750
137.30338
-4.8466422
-3802.8
4.38
124.94
PSP_001488_1750
137.30307
-4.8457244
-3815.7
4.51
111.27
PSP_001488_1750
137.30442
-4.8455009
-3799.2
2.34
79.11
PSP_001488_1750
137.33242
-4.8638848
-3507.1
2.04
132.56
PSP_001488_1750
137.31164
-4.9369596
-3278.4
3.74
129.01
PSP_001488_1750
137.30984
-4.9389356
-3287.6
4.46
119.79
PSP_001488_1750
137.32216
-4.9201255
-3290.9
1.79
134.69
PSP_001488_1750
137.31702
-4.922859
-3306.6
4.20
160.7
PSP_001488_1750
137.33822
-4.901912
-3265.6
4.87
-174.83
PSP_001488_1750
137.3328
-4.8924006
-3389.9
6.71
117.93
PSP_001488_1750
137.3421
-4.8631512
-3464.4
5.66
105.35
PSP_001488_1750
423
137.40969
-4.7799276
-3656.9
3.69
167.09
PSP_009149_1750
137.40533
-4.7770286
-3736.8
2.13
168.14
PSP_009149_1750
137.43867
-4.7530302
-3810.1
6.07
128.67
PSP_009149_1750
137.4381
-4.7521312
-3823.7
5.92
123.62
PSP_009149_1750